Among apparatus for recording and readout infra-red radiation images, it is known to use solid-state apparatus which primarily comprises a semiconductor substrate, for example of type N, which is covered on one side by a thin insulating layer and on the other side by a conducting metal layer. Transparent metal electrodes are regularly disposed on the insulating layer, for example in the form of a matrix comprising rows and forming columns and, with the insulating layer and the underlying semiconductor material, a corresponding number of elementary sensors or cells for sensing infra-red radiation which is directed onto the electrodes by means of a suitable optical device. Each of the electrodes and the metal layer of the substrate comprises an ohmic contact which permits electrical voltages to be applied thereto. By using suitable electrical voltages, it is possible to create, in the semiconductor, under the insulation of each of the transparent electrodes, a region of very small thickness, in which the free carriers are less numerous than in the rest of the semiconductor member. That "depopulated" region can then receive charges which are generated by the absorption of photons received by way of the transparent electrodes. The amount of charge which is accumulated in the above-indicated manner is then directly related to the amount of radiation received by way of the electrode. Changing the voltages applied to the contacts of the transparent electrodes and the metal layer of the substrate makes it possible to suppress the depopulated regions, giving rise to a discharge current which flows between the electrode and the metal layer of the substrate. By measuring that current, it is possible to determine the amount of accumulated electric charge and therefore the amount of infra-red radiation received.
In solid-state apparatus of the above-described type, the dimensions of each transparent electrode which corresponds to an elementary cell or an elementary sensor are of the order of 50 microns, and a large number of elementary cells therefore have to be grouped and assembled together in order to produce a screen and an image, or suitable dimensions. However, the number of cells is limited which can be grouped together and assembled, for example in the form of a matrix comprising rows and columns.
In fact, when considering a screen comprising 1024 cells and formed by a 32.times.32 matrix with a row and column pitch of 50 microns, the recording time would be a millisecond, in the case of infra-red radiation, which would mean that the reading time would be about one microsecond per cell, which is impossible to achieve at the present time.
It has therefore been proposed that the cells should be grouped for the reading operations, and that each group of cells should be read sequentially. Thus, in a 32.times.32 matrix, the 32 cells in a row are read simultaneously by applying an electrical signal thereto, by way of a row conductor which is connected to all the cells in a row; the readout signals appear on the 32 column conductors, each column conductor being connected to the cell of a column. That lay-out is satisfactory when the number of cells connected to the same column conductor is between 32 and 64. In fact, on the one hand, each column conductor has one of its ends connected to an amplifier. The pass band and therefore the noise of the amplifiers increases in proportion to the frequency of the signals read. The signal frequency will increase in proportion to an increase in the number of cells in a column. On the other hand, the signal of each elementary cell of a column, which will be applied to the amplifier, will decrease in strength in proportion to an increase in the number of cells in a column, as a result of the attenuating effect due to the capacitances of the cells of the column which are not selected for the reading operation. Those two phenomena act in opposition to each other, in dependence on the number of cells in a column, determining a number of cells, above which the signal read is weaker than the noise of the amplifier and cannot therefore be detected. At the present time, that number is from 32 to 64, depending on the matrices produced and the characteristics of the amplifier.
The above-described matrix lay-out, involving grouping cells in a row, also suffers from a limitation in regard to the number of cells in a row, due to the distributed time constant of the elementary cells, which limits the time for access to a cell. However, that limitation is less than that due to the number of cells in a column.
Due to the above-mentioned limitations, it is not possible to produce apparatus for recording and reading out images in the infrared spectrum, which would give images in a television format, for example on 625 lines, in such a way that the infra-red image can be transferred onto a television screen by line scanning. In fact, in order to carry out such a transfer operation, the apparatus for recording and reading images in the infra-red spectrum would have to comprise 625 elementary cells per column, whereas the abovementioned limitation is between 32 and 64.